The Port of London Authority has underlined its “serious concerns” about proposals for a Thames Estuary airport.

Speaking to boat clubs and other river users at a meeting on Canvey Island, PLA Chief Executive Richard Everitt, said the authority had two main worries.

One was any kind of barrage into the river, the other was any effect on the river’s shipping lanes.

He said; “We have no idea of the effect on shipping lanes, silting up and so on. It would need a huge amount to convince us.”

The PLA has previously said “The protection of the shipping lanes that service the port is paramount, given its national strategic importance. These shipping lanes also serve the Port of Medway. The Thames, in combination with Medway, makes up the largest port area in the UK.

“We would aim to ensure we maintain or improve the efficiency of London as a port, currently employing over 46,000 and generating over £3bn in value.”

Meanwhile campaigners fighting plans to build the airport off the Kent coast staged a protest outside City Hall in London.

It followed Boris Johnson's renewed call for an estuary airport telling David Cameron a go-ahead would be “a moment of greatness.”

Kent County Council has published its own vision of aviation calling for more use of existing airports such as Heathrow.

Two plans have been put forward for airports in the Thames estuary - one on an artificial island, dubbed Boris Island, and another by architect Norman Foster who proposes building a £150m airport off the Isle of Grain.